To investigate the hydrogen bonding and its dynamics of N-heterocyclic
carbenes (NHCs) in solution, a molecular mechanical force field was
fitted for the homologous series of 1,3-dialkylimidazol-2-ylidenes.
During the exploration of the potential energy surface of the
water/1,3-dimethylimidazol-2-ylidene system, it was observed for the
first time that the carbene is prone to interaction with hydrogen bond
donor molecules also from the rather unusual "on top" orientation, where
the direction of the interplay is perpendicular to the plane of the
NHC's ring. The fitting of the force field parameters for
imidazol-2-ylidenes was found to be the best in the case of a two-site
model, which reproduces not only the strength, but also the direction
dependency of hydrogen bonding. With the aid of this tool, curious,
hitherto unknown types of hydrogen bonding could be unveiled for NHCs.
In the case of non-hydrogen bonding solvents, carbenes tend to form
short lived, but structurally influental hydrogen bonds between each
other via ring hydrogen atoms and the divalent carbon atoms. The
chemically highly important hydrogen bond dynamics of NHCs was found to
be facilitated by three center hydrogen bonding, where two alcohol
molecules bind to a carbene, which is allowed only by the aforementioned
relatively strong interaction between the NHC and the hydrogen bond
donor in the "on top" orientation. The latter finding has significant
effects on processes that involve this kind of replacement, such as the
selective transesterification reactions, and the mechanism of proton
exchange on azolium rings.